Rotary filters have been widely used in the chemical industries for many years. It is typical of such filters that the solid particles which are laid down on the filter drum must be washed in order to remove residual liquid remaining therein. It has been found extremely difficult to apply wash liquid satisfactorily and consequently, such efforts have been the subject of much prior art.
In U.S. Pat. No. 949,724 an early means of disposing of wash liquid on a filter cake is disclosed to be simply a trough suspended above a rotary filter drum with liquid overflowing a curved lip on one side of the trough. This simple technique would appear to be satisfactory, but in fact is found to be subject to substantial difficulties owing to the need for absolute leveling of such distribution troughs, especially when they are of substantial length. For example, a commercial filter drum may be 15 or 20 feet in length, so that only a slight deviation from the horizontal position would cause great disparity in wash rate from one end of the cake to another.
An improvement on the simple trough is illustrated by U.S. Pat. No. 1,059,327, wherein an overflow trough has been provided with V-shaped notches to create a weir-type distributor. Although the weir device will overcome some of the disadvantages of the overflow lip of the earlier patent, nevertheless it remains sensitive to the horizontal leveling of the trough.
For some purposes such trough systems may provide a satisfactory result. It may be inferred from the prior art, however, that improved distribution was required since more complex belt distributors were introduced. Such systems are typified by U.S. Pat. Nos. 1,475,244, 2,092,111, and 2,202,932. These devices have as their objective uniform distribution of wash liquid by passing it through a porous belt, whereby the liquid is spread out onto the cake to provide a thin film of liquid. At the same time, to assure uniformity in the wash rate, the cake was leveled by the belt. While such devices would seem to be satisfactory for some materials, if the cake is subject to blinding by scraping it, it is quite probable that a belt distributor would cause a reduction rather than an improvement in wash liquid distribution since the passages in the cake could be blocked by the rubbing action of the belt.
U.S. Pat. No. 2,698,687, shows a compartmented belt holding in its individual segments significant amounts of liquid, which presumably would be confined to the area of the cake directly beneath it.
U.S. Pat. No. 3,215,277 illustrates a more recent technique whereby capillary resistance is used to provide distribution between the two sets of openings in a distributor. Liquids issuing from the first set of openings is passed to the second set through a medium which provides significant resistance to its passage.
U.S. Pat. No. 3,729,414 illustrates a distribution system for a rotary filter in a process similar to that for which the present invention was developed. In that system, distribution pipes are fed with liquid from several entry points and the liquid flows outwardly through small holes in the pipes and runs down and around grooving and drips off the bottom of the pipe. In such a distributor only a very small pressure can be used since the liquid must flow around the pipe rather than jetting out of the small holes. Thus, since very little pressure can be applied to the distributor pipe in the invention, it is not possible to obtain optimum distribution along the pipe and distribution will be disturbed by changes in wash flow rates.
The process to which U.S. Pat. No. 3,729,414 was directed is a process for removal of wax crystals from lubricating oils. In that process, oil is dissolved in solvent and chilled to form wax crystals which are then removed by filtration. As will be appreciated, the wax crystals are soft and consequently the spaces between the crystals may be easily blinded by physical contact. As a result, belt distributor systems are unsatisfactory since they would have a tendency to blind the cake and thus create uneven distribution through it. If the wash liquid leaves the distributor pipes at a high velocity, it can impact against the cake with sufficient force to dislodge it or to cut channels in it and thereby prevent uniform washing.
Such filters can also be provided with spray nozzles mounted on distributor pipes. Since spray nozzles create relatively high velocity sprays, reasonably even distribution of liquid is possible at the outlet of the spray nozzles. However, the high velocities with which the spray issues have detrimental effects on the cake porosity and are undesirable. At the same time wash rates can vary widely depending on the nature of the wax crystals. When this occurs, the shape of the spray will change with the liquid pressure and coverage by the spray nozzles will be dependent upon the wash rate. Accordingly, at low wash rates poor coverage of the cake and poor washing often occurs.
The present invention has addressed itself to the problems which have been discussed. In the wax removal process which is described generally in U.S. Pat. Nos. 3,773,650 and 3,779,894 relatively high wash rates are possible. The crystals which are produced by this wax producing process are substantially spherical in nature and consequently the filter cake is more porous than in prior art processes and thus the filter cake can and should take a significantly higher wash rate. Thus, it has become even more important than heretofore to obtain a uniform distribution of wash without imparting high velocities to the wash liquid which could cause damage or blinding of the cake. The present invention has satisfactorily solved these problems and is disclosed in the detailed description which follows.